Interpretive Summary: The expression and timing of female flowering in cucumber is an important component of uniform, stable fruit yield in cucumber. Female flowering is affected by the genetics of the plant as well as the environment in which the plant exists. Environmental stresses such as high or low growing temperatures and drought can cause female flowering plants to become male flowering, thus decreasing yield. There is a major gene that controls female flowering and its effect is modified by other genes which are controlled by environmental conditions. This research has identified the position of the female flowering gene or a modifier of female flowering. Knowledge of the location of this gene will allow plant breeders to better manipulate the control of flowering in cucumber.

Technical Abstract:
Sex determination in cucumber (Cucumis sativus L.) is controlled by three genes, F, m and a. Ethylene simulates the F locus for determination of sex in cucumber. Factors which induce ethylene biosynthesis, such ACC and auxin, simultaneously enhance female sex expression. A genomic sequence (CS-ACS1) encoding ACC synthase was amplified from genomic DNA by PCR using degenerate oligonucleotide primers. The deduced amino acid sequence (264 aa) is highly homologous (93% identity) to auxin-induced CM-ACS2 polypeptide in winter squash Expression of CS-ACS1 is positively regulated by auxin, but is not inducible by ACC, in apices treated in vitro or in planta. Southern blot hybridization analysis of several pairs of nearly-isogenic gynoecious and monoecious lines has revealed the existence of an additional ACC synthase (CS-ACS1G) sequence in the gynoecious lines examined. Sex phenotype analysis of a segregating F2 population detected a 100% correlation between the CS-ACS1G marker and the presence of the F locus. Thus, the CS-ACS1G gene is located in linkage group B coincident with the F locus. Collectively, these data suggest that CS-ACS1G might either be identified as the F locus itself, or a closely linked female modifier, and thus may play a pivotal role in determination of sex in cucumber flowers.